Nearly all natural communities exhibit spatial structure, particularly those composed of sessile organisms like forests. To this effect, spatial heterogeneity is thought to play an important role in the maintenance of species coexistence. Recent work indicates that local feedbacks – processes in which the presence or density of an individual organism modifies the local environment – are a particularly important source of spatial heterogeneity. Here, I study mathematical models that depict how local density-dependent interactions in spatially structured communities affect plant competition, community dynamics, and species richness. First, I examine how Janzen-Connell effects (distance-dependent specialized predation pressure) affects tree species richness. Recent theory indicates that Janzen-Connell effects may be unable to maintain diversity when realistic levels of fitness variation are considered. I demonstrate that the ability of Janzen-Connell effects to maintain species richness largely depends on how density-dependent interactions occur in space. Second, I extend the above framework to examine the interaction between local biotically-generated negative density-dependent feedbacks (e.g. Janzen-Connell effects) and abiotically generated spatial heterogeneity (e.g. soil topography). I demonstrate that biotically and abiotically generated spatial heterogeneity can strongly interact to shape species richness, particularly when the latter exhibits positive autocorrelation. Third, I examine how juvenile plant demography impacts the strength of negative density dependence. I analyze a stage-structured model and show that density-independent demographic rates strongly influence the strength and measurement of density-dependence. These results provide insight into empirical measurements of negative density-dependence, particularly recent results indicating that the strength of negative density dependence varies on environmental gradients. Fourth, I extend this lens of analysis to examine how temporal variation in seed availability impacts generalist predators, localized specialist predators, and tree coexistence. Many perennial trees exhibit masting, irregular periodic intra-specific synchronous production of seeds. Masting is thought to an evolutionary adaption to satiate predators. However, previous theory does not examine how masting affects the ability of localized predators to maintain species richness. I show that masting can be stabilizing (increases species richness) or destabilizing (decreases species richness) depending on the relatively non-linearity of specialist and generalist functional responses. Fifth, and in conclusion, I review these results in the context of similar models, highlight existing theoretical gaps, synthesize theoretical results from the literature, and point toward ways to better integrate empirical and theoretical work related to density-dependent feedbacks in spatially structured communities.